1. Field of the Invention
This invention relates to amplifiers and more particularly to amplifiers operable in relatively high noise environments wherein the noise received by the amplifier must be substantially reduced to provide a suitable signal to noise ratio.
2. Description of the Prior Art
The sensing of magnetic bubble domains at a particular point in a magnetic bubble chip has been accomplished in the prior art by placement of a magnetoresistive sensor at the point in the chip at which sensing is to occur. A current source is connected in parallel with the magnetoresistive sensor to provide a fixed current flow through the sensor. The magnetic flux attributable to the presence of a magnetic bubble closely adjacent the magnetoresistive sensor provides a change in the resistance of the sensor such that a different voltage is present across the sensor terminals than is present in the absence of a magnetic bubble domain.
The resistance variation depending on the presence or absence of a magnetic bubble domain closely adjacent a magnetoresistive sensor is small. For example, a resistance variation of 1 percent is not unusual. Further, the fixed current that is passed through the sensor is limited to a relatively low value both to avoid electromigration and to minimize the magnetic field produced by the current flow itself, which would, otherwise, override the field provided by the magnetic bubble domain unless held to a negligible intensity by limiting the current flow through the magnetoresistive sensor.
The voltage variation between the presence and absence of magnetic bubble domains in close proximity to the magnetoresistive sensor is the product of the current flowing through the sensor and the resistance variation of the sensor. As a result of the limited resistance variation and the limited current flowing through the sensor, as described above, the difference in voltage across the sensor depending on the presence or absence of magnetic bubble domains closely adjacent the sensor is rather small.
The small signal output from the magnetoresistive sensor must, therefore, be amplified to provide signal levels compatible with conventional logic circuitry. In magnetic bubble memory systems a major problem in amplifying the small output signal from the magnetoresistive sensors is that the signal leads are subjected to the noise introduced by the time varying, rotating magnetic field used to bias the magnetic bubble chip. The direction of this magnetic field is parallel to the magnetic bubble chip and to the substrate; therefore, if conductors are looped from the top surface of the chip down to the top surface of the substrate, the loops form an area that is perpendicular to the time varying magnetic field which allows maximum pickup of noise on these signal output leads. One solution to this problem has been to mount a sense amplifier closely adjacent the magnetoresistive sensors on the magnetic bubble chip to amplify the sensing signals to a relatively high level before the signals are brought out of the area of the time varying magnetic field. Two advantages of this approach are that 1) the signals are at a relatively high level when the noise component is added to them by virtue of the loops and 2) a relatively smaller amount of noise is introduced into the signal from the magnetoresistive sensors before amplification because the area of the loops of wires between the magnetic bubble chip and the sense amplifier chip that are perpendicular to the time varying magnetic field may be minimized.
It is desirable to reduce any D.C. offset at the input and at the output of the amplifier. This can be accomplished by use of relatively expensive amplifier circuitry or by use of relatively inexpensive amplifier circuitry with a feedback loop including another amplifier having an input connected to the output of the sense amplifier and an output connected to the input of the sense amplifier. A bypass capacitor is used at the input of the feedback amplifier to present a low impedance path to ground for the high frequency bubble domain sense signal so that only the D.C. offset component of the sense amplifier output signal is amplified by the feedback amplifier and introduced back into the input of the sense amplifier. In an integrated circuit implementation of the sense amplifier and feedback loop, the capacitor must be mounted off of the amplifier integrated circuit chip. In mounting the capacitors off of the amplifier chip, loops of wire subject to noise pick-up must be installed between the surface of the amplifier chip and the substrate. The noise picked up by these loops is then introduced into the input of the feedback amplifier, is amplified, and is then introduced into an input of the sense amplifier whereupon the noise is then amplified by a factor equal to the gain of the sense amplifier. This, of course, may result in the presence of noise in the output signal that is of the same order of magnitude as the amplified sense signal.
It would, therefore, be desirable to amplify relatively small signals by utilization of an amplifier including a feedback loop, whereby noise introduced into the amplifier by the feedback loop is cancelled to inhibit amplification of this noise.